Pacidesmus tuachua, Nguyen & Vu & Eguchi, 2024

Nguyen, Anh D., Vu, Tam T. T. & Eguchi, Katsuyuki, 2024, The millipede family Polydesmidae Leach, 1816 (Diplopoda, Polydesmida) from Vietnam, with a description of a new cavernicolous species, ZooKeys 1190, pp. 259-280 : 259

Pacidesmus tuachua sp. nov.

Figs 1 , 9 , 10 , 11 , 12 , 13 , 14

Materials examined.

Holotype. Vietnam • ♂; Dien Bien Province, Tua Chua District, Xa Nhe commune, Xa Nhe cave ; 600 m a.s.l.; 21°52'37"N, 103°24'48"E; 12 April 2022; Anh D. Nguyen leg.; IEBR-Myr 951H.

Paratypes. Vietnam - Dien Bien Province • 5 ♂♂, 10 ♀♀; Tua Chua District, Xa Nhe commune, Kho Chua La cave ; 600 m a.s.l.; 21°52'36.9"N, 103°24'47.9"E; 12 January 2021; Anh D. Nguyen leg.; IEBR-Myr 899 • 3 ♂♂, 4 ♀♀; Tua Chua District, Xa Nhe commune, Kho Chua La cave ; 600 m a.s.l.; 21°52'36.9"N, 103°24'47.9"E; 12 January 2021; Anh D. Nguyen leg.; IEBR-Myr 900 • 4 ♂♂, 5 ♀♀; Tua Chua District, Xa Nhe commune, Kho Chua La cave ; 600 m a.s.l.; 21°52'36.9"N, 103°24'47.9"E; 12 April 2022; Anh D. Nguyen leg.; IEBR-Myr 951P • 4 ♂♂, 5 ♀♀; Tua Chua District, Xa Nhe commune, Xa Nhe cave ; 600 m a.s.l.; 21°52'37"N, 103°24'48"E; 12 April 2022; Anh D. Nguyen leg.; IEBR-Myr 952 • 2 ♂♂, 4 ♀♀; Tua Chua District, Xa Nhe commune, Kho Chua La cave ; 600 m a.s.l.; 21°52'36.9"N, 103°24'47.9"E; 12 April 2022; Anh D. Nguyen leg.; IEBR-Myr 953 .

Diagnosis.

The new species can be distinguished from its congeners by a combination of the following features: unpigmented colouration, small size (midbody width <4.0 mm), head narrower than collum, absence of sphaerotrichomes, lateral budges on male prefemora, subfalcate gonopod telopodite, absence of exomere, endomere with an acute triangular process distally and a broad triangular process medially, and endomere tip slightly and unequally bifid.

The species is truly cavernicolous, characterized by white or unpigmented colour and living within a cave. As a troglobiont species, it groups with all 12 troglobiont or troglophile congeners from China (Table 1 ). However, this species differs from all of these, except P. bifidus from the Hengli Xin Don Cave, Guangxi Province in the absence of an exomere and the gonopod telopodite showing no additional processes; the endomere also has two additional processes, and the tip of the endomere bears two tiny teeth. The new species is similar to P. bifidus in having a troglomorphic appearance, the absence of an exomere, and a bifid tip of the endomere, but it differs in having two tiny teeth at the tip of the endomere tip (vs two long processes in P. bifidus ).

The new species is assigned to Pacidesmus because of the following characters: the seminal groove starts mesally, then recurves laterad at the base of a particularly prominent endomere branch to enter an accessory seminal chamber that opens on a setose pulvillus; endomere bears additional processes.

Etymology.

The specific epithet is treated as a noun in apposition and is based on the "Tua Chua" district where the two caves are located.

Description.

Holotype length ca 16.3 mm, width of midbody pro- and metazonae 1.0 mm and 1.5 mm, respectively. In width, head <collum <segment 3 = 4 <2 <5 = 15, thereafter body gradually tapering towards telson (Figs 9 , 10 , 12 ). Colouration in alcohol rather uniformly white (Figs 8 , 9 ). Body with 20 segments. Antennae long and only slightly clavate, possibly reaching past segment 3 if stretching laterally; antennomere 3 longest, approximately 1.3 × longer than subequal antennomeres 4-6; antennomeres 5 and 6 each with a small, compact, distodorsal group of bacilliform sensilla; antennomere 7 with a minute dorsoparabasal cone and a distodorsal group of microscopic sensilla (Figs 9A-C , 11C ).

Paraterga (Figs 9 , 10 , 12A-C ) strongly developed, set high, starting with collum, dorsum faintly convex; paraterga mostly weakly upturned above dorsum. Caudolateral corner of paraterga acute, postcollum ones extending increasingly past rear tergal margin, especially so in segments 16-18. All poreless segments with three incisions, all pore-bearing ones with four minute incisions at lateral margin. Front margins of metaterga narrowly bordered and forming distinct shoulders.

Ozopores evident, dorsal, located in front of posteriormost marginal incision of paraterga 5, 7, 9, 10, 12-13, 15-19.

Metatergal sculpture typical, poorly developed, obliterate, with three transverse rows of typical (= polydesmid), setigerous, polygonal bosses. Tergal setae short, slightly longer only on collum, simple, often obliterate. Stricture between pro- and metazona wide, shallow and nearly smooth. Limbus exceedingly thin, microdenticulate (Fig. 12A-C ). Pleurosternal carinae absent.

Epiproct (Figs 11A, B , 12C, D ) short, conical, pre-apical lateral papillae evident. Hypoproct (Figs 11A , 12D ) subtriangular; distolateral setiferous knobs small, but distinct and well separated.

Sterna without modifications, but setose (Fig. 10A, C ).

Legs generally long and slender, apparently slightly incrassate, approximately 1.7-1.8 × as long as midbody height, densely setose, almost all setae simple, poorly branching setae with minute, distal, side branchlets only on slender prefemora, latter devoid of lateral bulges.

Gonopods (Figs 11D , 12 , 13 ) characteristically subfalcate (vs suberect in all other congeners). with large, rectangular coxites (co), with a few long setae ventrally; a long, simple, flagelliform cannula (ca) as usual. Telopodite elongate, stout, strongly falcate or C-shaped; prefemorite (pref) densely setose; seminal groove starting mesally, then recurving laterad to run to the opening on a hairy pulvillus. Endomere (en) with two additional processes, a shorter, larger, broader triangular process at midlength (p1), and a longer, acuter, triangular process at ¾ length (p2). Tip of endomere unequally bifid, a longer and a shorter branch. Neither an exomere nor a clivus.

Female. Slightly larger than male, length ca 16.8 mm, width of pro- and metazona about 1.1 mm and 1.7 mm, respectively. Paraterga slightly less strongly developed. Legs unmodified, somewhat shorter and more slender. Vulvae highly elevated. Epigynal ridge low.

DNA barcode.

Two COI sequences (661 bp) were uploaded to the GenBank with the accession numbers PP118040 and PP118041. The new species has a close COI identity to Epanerchodus koreanus (NC051495) at 88.72% (query coverage 97%).

Habitat.

This species is to be considered a true troglobiont because it shows the typical morphological features of a cave-dweller. It was collected exclusively in the dark zone of the caves as described below. Kho Chua La and Xa Nhe caves are both located close together, approximately 500 m in distance. These caves are at the centre of the Xa Nhe commune, Dien Bien Province, northwestern Vietnam. The two caves are tunnel-like: they are high (15-20 m), wide (15-20 m), and long (1,000-1,500 m). The floor is mainly wet, with clay, and some small pools. Several other millipede species have been found in these caves, including Glyphiulus sp. ( Spirostreptida , Glyphiulidae ) and Eutrichodesmus sp. ( Polydesmida ,: Haplodesmidae ). The new species was found>1000 m from the entrance.

Kho Chua La and Xa Nhe caves are located on the Tua Chua karst plateau in northeastern Dien Bien Province, northwestern Vietnam. The natural area is about 68,414 ha, and 70% of this area is composed of limestone mountains, which are known for their layers of majestic rugged rock and unique natural landscape. The karst region contains many stunning and well-known caves, such as Kho Chua La, Tham Khem, Hau Chua, Xa Nhe, and Pe Rang Ki ( Nguyen et al. 2022). Furthermore, the Tua Chua karst plateau of northwestern Vietnam is close to the Yunnan and Sichuan karst regions of southwestern China. Given this, it is not surprising to discover the genus Pacidesmus in northwestern Vietnam. The distance between Pacidesmus tuachua sp. nov. and Pacidesmus trilobatus Liu & Golovatch, 2020 from Guangxi Province, China, is about 150 km northeast-southwest (Fig. 1 ).

Remarks.

While there remains a noticeable geographical gap between the mountainous northern Thailand species and the troglobionts of southern China, the discovery of a new species in northern Vietnam partially fills this gap. Like the more eastern species, the new species is also troglobiotic, and whether these species should be classified in a distinct genus related to a more restricted Pacidesmus (including only the type species) needs exploration.

The discovery of a new species marks the first record of the genus Pacidesmus in Vietnam.

Figures

Figure 1. Records of polydesmid species in Vietnam and of all known Pacidesmus species.

Figure 8. Polydesmus vietnamicus Nguyen, 2009 from Ha Giang Province (IEBR-Myr 808) ♂, right gonopod A mesal view B lateral view. Abbreviations: co = coxite; pref = prefemorite; ca = cannula; en = endomere; sl = solenomere; mt = midway teeth; ap = additional process. Scale bars: 0.5 mm.

Figure 9. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) A anterior part of body, dorsal view B anterior part of body, lateral view C anterior part of body, ventral view D segments 8 - 10, dorsal view. Scale bars: 1 mm.

Figure 10. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) A segments 8 - 10, ventral view B-D posterior part of body, dorsal, vental and lateral views, respectively. Scale bars: 1 mm.

Figure 11. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) A telson, ventral view B telson, lateral view C right antenna, anterior view D gonopods in situ under UV light, ventral view. Scale bars: 1 mm.

Figure 12. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) A anterior part of body, under the UV light, dorsal view B segments 8 - 10, dorsal view C posterior part of body, dorsal view D posterior part of body, ventral view. Scale bars: 1 mm.

Figure 13. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) right gonopod, under UV light A mesal view B ventral view C lateral view D dorsal view. Scale bars: 1 mm.

Figure 14. Pacidesmus tuachua sp. nov., holotype ♂ (IEBR-Myr 951) right gonopod A mesal view B lateral view. Abbreviations: co = coxite; pref = prefemorite; ca = cannula; en = endomere; pu = puvillus; p 1 = first process; p 2 = second process. Scale bars: 0.5 mm.